The production of water and hydrocarbon fluids from subterranean formations reduces reservoir pressure and removes fluids from the interstitial pores of the subterranean formation. The reduction in pressure and fluid loss may cause subsidence and compaction of the subterranean formation, and the risk increases when the formation has relatively high porosity and a low compressive strength.
It is difficult to observe the compaction of a hydrocarbon reservoir, but subsidence at the surface is often easy to see. For example, water can encroach on previously dry land, an offshore platform can lose its air gap between the high waves and the bottom deck, wellheads and casing may begin to protrude from the surface, or surface structures can sink.
An excessive amount of subsidence may result in well casing failure or rig collapse, affect cap rock integrity and can permanently damage the permeability and hydrocarbon producing capability of a subterranean formation if the interstitial pores are irreparably closed. The economic consequences of compaction and subsidence can thus be huge, but not all of the consequences are negative. Compaction can also be beneficial, as it provides a potentially strong production-drive mechanism. In either case, it is desirable to monitor the subterranean formation to detect the onset of subsidence and compaction in order to effectively manage the reservoir.
In addition to subsidence and compaction, the opposite changes can also occur. Where high volumes of water, gas, and/or steam are injected into a reservoir, rock formations can dilate, thus causing surface heave. Excessive surface heave and reservoir dilation also pose a risk to surface facilities, well casing failures, and cap rock integrity. Simulation studies also suggest that reservoir dilation can increase reservoir porosity and permeability. Unfortunately, however, there are no existing technologies to date that can measure this effect in the reservoir.
Deformation monitoring methods differ for onshore and offshore areas. Onshore, benchmarks are common tools of civil engineers. A benchmark is a survey mark at a known position and a measured elevation that is used to determine changes in elevation with respect to other benchmarks. Benchmarks outside of the subsidence bowl provide fixed reference points.
The most accurate way to determine an elevation difference between benchmarks is to connect two locations with a liquid-filled tube. The hydrostatic level will be the same at both ends of the tube, so changes in relative elevation can be determined with great accuracy. However, performing this type of survey over large areas can be prohibitively expensive.
Tiltmeters—devices that are sensitive to the change of angle on the surface or in wells—can provide subsidence data for onshore locations. These devices are also used to monitor the advance of an induced fracture.
Global positioning system (GPS) stations can be used for fixed positions either onshore or offshore. Under ideal conditions, GPS techniques can detect elevation changes of about 2 mm.
Another method that is under evaluation by several companies uses satellites for subsidence monitoring. Interferometric synthetic aperture radar (InSAR) relies on repeated imaging of a given geographic location by air- or space-borne radar platforms. The InSAR method has limitations, though, because growth of vegetation between satellite passes can cause interpretation problems over open fields. Also, rapid changes in elevation, such as occur near active faults, are easier to measure than slow subsidence. Finally, distance measurements can be made when the satellite is ascending or when it is descending. Since the angle of reflection is different, the two measures generally involve different sets of scatterers, and the ascending and descending measurements of subsidence may not agree completely.
Offshore, the subsidence bowl is not easily accessed. Most commonly, subsidence is monitored at platforms. This is not merely a convenience, but a necessity. The air gap, or distance between mean sea level and the lowest structure of the platform, has to remain greater than the wave height. Companies use a statistically derived wave height, often the maximum wave height expected over a 100-year period.
The air gap can be measured by several methods, all of which rely on a known benchmark on the platform. Continuous measurement of distance to the water can be obtained acoustically; alternatively, an underwater pressure transducer mounted on the leg of the platform can indicate the height of the water column above it. Interpretation of these two methods requires knowledge of sea level at the time of the measurement, which means tides and wind-driven waves have to be considered.
Today, the most common method for determining platform subsidence is by using GPS, as is done onshore. Some interpretation methods require a nearby platform that is not subsiding, but the methodology is improving, and some companies that provide this service to the industry now claim their interpretation does not require a near, fixed benchmark.
Subsidence also affects pipelines and other structures on the seabed. Bathymetry surveys are the most direct way to map the extent of an undersea subsidence bowl. The survey indicates water depth with respect to sea level. This is generally obtained by bouncing an acoustic signal off the mudline and back to a receiver. The traveltime measurement must be corrected for the effects of water salinity and temperature, and variation therein can affect accuracy.
Heave is usually more difficult to measure than subsidence. In the past, time-lapse logging for casing collar location and for petrophysical markers, time-lapse seismic studies, and microseismic arrays were used. But these methods have largely been dis-continued as insufficiently accurate.
The radioactive marker technique (RMT) for in situ compaction measurements in deep producing gas/oil reservoirs was originally developed almost 40 years ago (De Loos, 1973, In-situ compaction measurements in Groningen observation wells, Verhandenlingen Kon. Ned. Geol. Mijnbouwk. Gen., 28, 79-104) and since then has continuously improved to become the most commonly used method for monitoring subsidence (Mobach and Gussinklo, 1994, In-situ reservoir compaction monitoring in the Groningen field. Proceedings of EUROCK 94, Rock Mechanics for Petroleum Engineering, The Netherlands, 535-547. A.A. Balkema Publ).
RMT provides a realistic estimate of the uniaxial vertical compressibility CM of producing gas/oil reservoirs. The RMT technique is based on repeated measurements of the vertical distance between weakly radioactive isotopes located into bullet-shaped steel containers (called markers) and shot about 10.5 m apart within the producing formation through the wall of a vertical, generally unproductive, well prior to the casing operations. The best place to put the markers is in a vertical monitor well because deviated wells introduce an error in the position of the marker, depending on the orientation of the gun when the bullets are fired. It is also best to avoid producing wells, since producing wells may also flow formation solids, introducing uncertainty about the cause of the marker movement—either compaction or solids production.
Once the markers are in place, their position can be determined over time to monitor deformations. Generally, an invar rod carrying two pairs of gamma-ray detectors within or thereon is slowly raised at a constant speed from the borehole bottom and records the count rate peaks when the detectors are facing the markers. The mean spacing between the top and bottom detectors is roughly the same as the spacing between the markers, which minimizes distance errors due to any tool movement from the wireline cable stretching and contracting. The recording procedure is typically repeated three to five times to minimize instrument and operational errors. Finally the measurements are processed to obtain an average estimate of the shortening Δhi of the i-th monitored interval.
If Δpi is the average pore pressure drawdown experienced by the formation where the i-th marker pair is located, the in situ uniaxial rock compressibility can be estimated as:
      c          M      ,      i        =            Δ      ⁢                          ⁢              h        i                            h        i            ⁢      Δ      ⁢                          ⁢              p        i            
with hi the initial marker spacing approximately equal to 10.5 m.
The field CM can also be evaluated by the simple equation:
      c    M    =                    Δ        ⁢                                  ⁢        h            _                                h          0                _            ⁢      Δ      ⁢                          ⁢      p      
where Δh= ht− h0 is the average vertical deformation (expansion if positive, compaction if negative) of the marker interval; h0 and ht is the average distance between two adjacent markers at the initial time and at time t, respectively; and Δp is the fluid pressure variation (rise if positive, drawdown if negative) that occurred within the monitored depth interval over the time period 0-t.
Additional details on these and various other stress and deformation calculations is readily available (e.g., M Ferronato, et al., Unloading-Reloading Uniaxial Compressibility Of Deep Reservoirs By Marker Measurements, Proceedings, 11th FIG Symposium on Deformation Measurements, Santorini, Greece, 2003, M Ferronato, et al., Radioactive Marker Measurements in Heterogeneous Reservoirs: Numerical Study, International Journal Of Geomechanics, 79-92 (2004) (incorporated by reference) and similar literature).
Unfortunately, although a very useful technique, the use of the radioactive markers poses an environmental exposure hazard under some circumstances. For example, when a subterranean formation comprises high density rock, the bullets may bounce and can become lost or partially lodged in the wall of the borehole. Other stray radioactive marker bullets may simply fall to the bottom of the borehole. These stray radioactive marker bullets are typically left in the borehole and cemented in. When the next section of the borehole is drilled, some of the stray radioactive marker bullets may be crushed, exposing the drilling mud to radioactive particles, risking human exposure or exposure to the external environment and/or aquifers. Accordingly, it would be desirable to use depth markers that do not pose such hazards to the environment, personnel or nearby residents.
Additionally, the RMT of the prior art only provide rough information about changes in the depth of the radioactive marker. Information about orientation and horizontal shift is not available. Thus, the information provided is incomplete.
Accordingly, there is a need in the art for improved methods, devices and system for monitoring subsurface formation deformation that address one or more disadvantages of the prior art.